Detection of specific target analytes, or chemical compounds, is important for many applications, including for example, detecting whether the concentration of analytes exceeds flammability limits. Target analytes are detected by sensors operating according to different detection mechanisms, known in the art. Most sensors employ a sensing component that is physically modified in the presence of specific analytes present in the environment. Thus, a sensor typically comprises a probe that includes both the sensing component and a probe body housing (including terminals for transmitting an output). The terminals are typically coupled to a processor, also part of the sensor, which analyzes the outputs received from the sensor probe. Such processor is coupled to a user interface, typically containing an indicating device, which signals when concentration of an analyte has exceeded threshold values.
Many sensors employ a sensing component that is a sensor film. Many sensor films swell, increasing in volume, while in the presence of the analytes. Various sensors available in the art utilize the physical changes in the sensor film to determine concentration of analyte present. Such sensors may include optical sensors, such as fiber optic sensors, where a beam of light is projected through an optical fiber at a sensor film cladding, and physical changes (e.g. refractive index or color) in the film are monitored. Such changes in refractive index occur when analytes are absorbed and change the physical properties of the cladding (including volumetric changes). Other sensors include sound acoustic wave sensors (SAWS), which project ultrasonic waves through the sensor film between transducers, and likewise detect any modifications in the properties of the sensor film (primarily the mass), translating those changes to the concentration of analyte present.
Another type of sensor film is a conductiometric sensor, more particularly, a polymer-absorption chemiresistor sensor. A polymer-absorption chemiresistor has a polymer film sensor exposed to a surrounding atmosphere containing target analytes (chemical compounds). An electrical charge is applied across the polymer film. The polymer absorbs target analytes and this results in a volumetric change of the film, and hence the electrical resistance of the film. Further, conductive particles may be distributed throughout the polymer film to enhance the sensitivity to resistance changes in the material when the volume of the polymer changes.
While conventional chemiresistor sensors perform adequately for their intended uses, they are subject to improvement. Specifically, as the volume of the sensor film expands and contracts over time in response to the presence of the target analytes, the mechanical bond between the sensor film and the terminals is weakened, thus causing the film to gradually separate from the terminals. As the film separates from the terminals the electrical bond between the film and the terminals is also weakened. This weakening of the electrical bond between the film and the terminals decreases sensor performance because it diminishes the ability of the processor to analyze changes in the resistance of the film through the terminals. Consequently, there exists a need for an improved chemiresistor that provides an enhanced mechanical and/or chemical bond between the terminals and the sensor film to enhance both the robustness of the sensor and the responsiveness of the sensor to the target analytes.
Conventional chemiresistor sensors are also deficient in that they fail to provide a sensor probe having a sensor film of a controlled thickness. The thickness of the sensor film is relevant to the probe's ability to detect the target analytes. Specifically, the use of a thick sensor film is undesirable because thick films require an extended period of time to absorb the target analyte, thus increasing the time required for the sensor film to swell and produce a change in resistance indicating the presence of the target analyte. However, the use of a sensor film that is overly thin is also not desirable because excessively thin films are not durable, are difficult to manufacture, and are unstable. Thus, there is also a need for a chemiresistor that has a sensor probe with a sensor film of a controlled thickness.
Additionally, a build-up of dirt or surface moisture may contaminate the terminals of conventional sensor probes, causing the sensor to produce inaccurate readings. Specifically, the surface moisture effectively creates a bypass resistor in parallel resistance with the sensor probe. This bypass resistor typically desensitizes the performance of the sensor. In particular, if the bypass resistance becomes small enough, the combined resistance of the bypass resistor in parallel with the sensor probe is influenced more by the bypass resistor, thus a very low level current will run into the sensor probe. Consequently, there is also a need for a chemiresistor sensor having a sensor probe with a circuit that is insensitive to bypass resistance.